There have been numerous attempts to provide instruments for characterizing telephone circuits so as to describe the structure and provisioning of a circuit.
Characterizing the structure and termination characteristics of a telephone wiring circuit are of primary importance to the diagnostic technician, since it is these properties that are often responsible for problems on a circuit. A poorly terminated circuit, a circuit with extraneous wiring, a circuit with grounded or broken wires, or a circuit that loses too much signal because it is too long are all identifiable by understanding the circuit's structure.
Prior to the apparatus described herein, a device known as the Time Domain Reflectometer (TDR), such as the Tektronix Corporation Model 1503, has come closest to achieving the desired goal of telephone circuit characterization. A TDR works by injecting a high amplitude electrical pulse into the wiring of a telephone circuit. An electrical phenomenon called reflectivity results in a portion of the original pulse being reflected back toward the source of the pulse wherever the impedance of the circuit changes, such as at a junction. As the pulse propagates down the line and eventually reaches and reflects from the end(s) of the wire, the returned reflections can be captured and plotted as a graphical trace. A trained and experienced technician can analyze the curves of the graphical trace to determine many of the circuit's characteristics.
In addition, practitioners in the art of Technical Surveillance and Countermeasures (TSCM) have chosen TDR as the preferred analysis device for identifying conditions in wiring that might represent wiretaps. That is, TDR is used to detect changes in the circuit impedance; these are compared to the presumed circuit structure to locate additional changes in impedance possibly due to wiretaps or the like.
The apparatus and methodology described herein uses Frequency Domain Reflectometry (FDR) (also referred to as Standing Wave Reflectometry (SWR)) to characterize the structure and provisioning of a circuit in a manner that significantly outperforms TDR. As opposed to the discrete pulses transmitted in TDR, FDR utilizes a continuous waveform (i.e. a series of pulses forming, for instance, a square wave, a simple or complex sinusoidal signal, or a “sawtooth” signal comprising a series of triangular pulses) to transmit electrical energy into the circuit under test. As in TDR, the signal is reflected at points in the circuit at which the impedance changes, and the reflections are analyzed to determine the structure of the circuit. The continuous signals employed in FDR are generally of much lower amplitude than that of the discrete pulses used in TDR, resulting in less chance of damage to sensitive electronics than is risked with TDR.
The significance of being able to use both waveforms comprising continuous streams of pulses and non-pulse waveforms (i.e., sinusoids) in FDR is that certain waveforms cannot be used to analyze circuits that are already carrying certain types of signal without interference. Specifically, digital telephone circuits that cannot be analyzed using TDR can now be characterized in situ, that is, while active, using FDR with the appropriate choice of waveform.
Furthermore, the ability of FDR to use multiple low amplitude waveforms allows the apparatus of the invention to be used for long-term security monitoring. These attributes permit characterization signals to be transmitted and monitored continuously without negatively affecting the equipment being protected, and without alerting users and/or perpetrators that protection is active. Accordingly the circuit can be monitored continuously, even during use. Prior art apparatus does not allow users of digital telephones, for example, to be assured that they are not being monitored.
The telephone industry segment that installs and maintains Digital Subscriber Link (DSL) circuits for Internet access has had great difficulty finding circuits of sufficient quality (no bridge taps over a certain length, no load coils, low signal loss) for conversion to DSL during initial installation, or as replacements during operations. The apparatus described herein can be used to initially characterize available circuits to determine if they are qualified for effective DSL communications, and for in situ characterization during normal operations.
Although there are several industrial devices that use FDR/SWR to test optical cables and coaxial cable in antenna systems, e.g., the Model LST-1700 sold by Acterna, of Germantown, Md., the literature and prior art known to the inventor does not describe the use of FDR as a characterizing methodology for telephone circuits.
There is no prior art documents or products known to the inventor that disclose apparatus or methods for characterizing active digital telephone circuits, nor for protecting active digital telephone circuits against wiretaps and eavesdropping.
Pending U.S. patent application Ser. No. 10/134,726 filed Apr. 30, 2002, by Vasquez and the present inventor, called “Characterization of Telephone Circuits” (the “'726 application”), from which priority is not being claimed hereunder, uses a primitive statistical sampling technique of a small portion of an FDR trace to calculate a characteristic number, referred to therein as the Vasquez number. (At the time of the '726 application, it was understood that the device operated on some sort of reflectometry principle, but it was not understood that the reflectometry principle was FDR, and thus FDR was not referred to as such in the '726 patent application.) The characteristic Vasquez number is defined in the '726 application as a unitless number. The Vasquez number thus presented has no practical component that could be used to indicate the nature of the differences between two circuits being compared, such as what is different between them, or the distance between the source of the stimulus and the reflecting impedance change.
Additionally, there are certain conditions where ambiguity in readings provided by the instrument described in the '726 application is a significant possibility, such that two distinctly different circuits can generate the same Vasquez number, leading a technician into a false sense of security.
Thus, although the instrument described in the '726 application has utility, further improvements are called for.